The use of heat exchangers in the form of air conditioning cooling towers, roof top boilers and solar collectors in temperature control systems employing heat transfer fluid is well known. As a common example the solar collector consists of one or more collector panels forming an array and having a glazing or window positioned for maximum exposure to the sun. Within the collector panels are solar energy absorbers designed to maximize the absorbtion and retention of solar energy. Although there are many different possible configurations and materials employed in solar collectors, most collectors share the common characteristic of having channels in the absorbers through which the heat transfer fluid is circulated to be heated and returned to be utilized or stored in one or more tanks.
In practice, the solar collectors are most commonly mounted in elevated positions, such as the roof of a house, in order to have unobstructed positional access to the sun, to utilize pre-existing structural support and to facilitate transmission of the heat transfer fluid. While most solar collectors utilize water as the heat transfer fluid, in climates where the ambient temperatures drop low enough to freeze the fluid within the solar collectors, steps must be taken to prevent the fluid from freezing in the collector. The expansion of a fluid such as water within the collectors upon freezing results in the rupture and destruction of the expensive collectors and necessitates costly replacement. Although anti-freeze additives or special heat transfer fluids are utilized in some systems to prevent the fluid from freezing, such additives or special fluids are expensive, only reduce the freezing point of the fluid to a lower temperature, may be poisonous and therefore not usable in applications such as solar water heaters, and in many instances require more costly associated system components such as special piping, valving, pumps, and double-walled heat exchangers.
Another approach to protecting solar collectors from freezing is to temporarily drain the collectors before the temperature drops below freezing. This requires a system adapted to normally circulate fluid through the collectors and selectively drain the fluid when the ambient temperature proximate the collectors is below a predetermined minimum safe temperature for the fluid. One approach includes the use of a valve positioned below the collector in conjunction with a vacuum breaker positioned at the top of the collector array. Such a valve normally allows collecting fluid to be circulated through the valve, to the collector and back through the valve to storage or use. Translation of the valve to a drain position interrupts all flow connections and establishes communication between the collector return line and a drain line. In order for the valve to effect evacuation of fluid from the collector array when the valve is in the drain position, the vacuum breaker must actuate to allow communication between the highest point of the sealed collectors and the atmosphere. If the vacuum breaker fails to actuate, the collectors will not drain even though the valve is in the drain position. Since the vacuum breaker is characteristically exposed to ambient temperature at the collector array there is a tendency for the vacuum breaker to freeze thus not allowing the collector to drain and resulting in freezing the fluid in the collector and the attendant damage or destruction of the collector.
In order to assure that the valve will drain the collectors, multiple vacuum breakers may in some instances be installed in the collector line, at least one of which breakers being installed other than proximate the collectors where it will not be exposed to freezing temperatures, as for example in an equipment or control room maintained well above freezing temperatures. The addition of the second vacuum breaker doubles the expense and inconvenience without absolutely insuring complete drain down. Thus, Applicant is not aware of the existence of a totally satisfactory system for employing water as the heat transfer medium in colder climates which easily and positively drains collectors during weather conditions which could cause collector damage.